Rein check (3)
I believe the answer is:
bit
'rein check' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for bit that I've seen before include "Data measurement unit" , "Unit of information" , "Small quantity or piece of metal for a horse" , "Part of 21 across" , "Binary digit; small piece" .)